In the art of producing melt-blown microfibers, a plurality of spaced, aligned hot melt strands of polymeric material, or the like, are extruded downwardly simultaneously directly into the elongated zone of confluence formed by a pair of heated, pressurized, angularly colliding gas (usually air) streams, each stream typically being in a flat, sheet-like configuration and being on a different, opposed side of such strand plurality. The gas streams break up the strands into fine, filamentous structures, and move such forwardly, so that a non-woven mat thereof is continuously laid down upon a moving surface. The U.S. Naval Research Laboratory, Washington, D.C. and Esso Research and Engineering Company, Baytown, Texas, have heretofore reported research and development work on such process.
In the process, it is believed desirable to have the two flattened gas streams employed be not only as nearly identical to each other as practical (as respects such variables as gas composition, gas temperature, gas pressure, gas volume, stream angle with respect to the forward direction in which the strand plurality is being extruded, and the like), but also as uniform as possible. Thus, with respect to an individual one of such pair of streams, it is very desirable to control and maintain uniformly such variables as temperature, pressure, velocity, eddy currents, and the like. Preferably, each gas stream has a temperature about equal to that of the temperature of the strands in one presently preferred mode of practice.
In prior art apparatus used for the practice of this process, a pipe was located along each side of a die head adapted to extrude such strand plurality, and an elongated, slotted orifice in each pipe permitted air to escape therefrom and pass against each opposed side of such strand plurality. To supply heated air to each one of such pipes, a plurality of conduits in adjacent spaced relationship to each other joined the outside upper side wall of each such pipe; this arrangement was sometimes nick-named by those skilled in the art "the pipe organ". Unfortunately, this arrangement is not particularly easy or economical to construct or even to maintain. In addition, this arrangement characteristically produces a non-uniform temperature gradient along the mouth of each slotted orifice, causing a patterned variation of "hot" and "cold" spots therealong, these gradient differences being so great as to commonly cause a "striped" effect to appear in a non-woven web of melt blown microfibers produced with such arrangement. Such stripes indicate sheet thickness variations transversely along the path of web generation, and these thickness variations in turn are believed to be caused by temperature and perhaps even pressure variations in air stream uniformity along individual stream longitudinal width. Precise, accurate, stable, uniform individual gas streams are difficult, and probably impossible, to achieve with such prior art apparatus.
So far as is known, no one has heretofore discovered a system for the gas stream generation required in practicing the melt blown microfiber process which is well suited for large scale industrial utilization, which has associated favorable cost, maintenance, long life, and reliability features, and offers the potential of overcoming disadvantages of prior art apparatus above described, so that gas stream characteristics may be equalized and made uniform before being impinged upon a plurality of hot melt strands to be attenuated.